Terrestrial digital radio and TV (DAB and DVB-T) are transmitted using digital multi-carrier methods (e.g. OFDM=orthogonal frequency division multiplexing) via a network of transmitters, which transmit within the transmission range in a phase-synchronous and frequency-synchronous manner via a single-frequency network.
For an efficient exploitation of the available frequency resources, all the transmitters of a single-frequency network simultaneously transmit an identical transmission signal. In addition to phase synchronicity, the identity of the carrier frequency to be transmitted in the individual transmitters must therefore also be guaranteed within a single-frequency network.
German published patent application no. DE 199 37 457 A1 discloses a method for monitoring the phase synchronicity of individual transmitters of a single-frequency network. The occurrence of a phase synchronicity of two transmitters is registered via a measurement of propagation-time difference by determining the channel impulse responses of both of the transmitters. If a large-scale deviation between the measured propagation-time difference of the two transmitters and a reference propagation-time difference for synchronous operation of the two transmitters is registered, then the transmitters are transmitting in an asynchronous manner. This deviation in the propagation-time difference is determined by a receiving station within the transmission range of the single-frequency network by evaluating the channel impulse responses and communicated to the two phase-asynchronous transmitters to allow subsequent synchronisation. A method for monitoring identical carrier frequencies in two transmitters within a single-frequency network is not disclosed in DE 199 37 457.
The synchronisation of transmitters in a single-frequency network with regard to an identical carrier frequency is described in German published patent application no. DE 43 41 211 C1. In this context, alongside the transmission data, a central system also transmits a frequency reference symbol to the individual transmitters of the single-frequency network. This frequency reference symbol is evaluated by every transmitter in the single-frequency network and is used to synchronise the carrier frequency with the reference frequency.
The disadvantage with this method is the fact that the synchronicity of the carrier frequency is evaluated by each transmitter individually. Accordingly, this transmitter-specific evaluation of the frequency synchronicity of the carrier frequency may be associated with a certain transmitter-specific measurement and evaluation error, which can lead to a non-uniform monitoring of the carrier frequencies of all the transmitters participating in the single-frequency network. Added to this is the fact that the monitoring of the carrier frequency in each individual transmitter necessitates a synchronisation of the individual transmitters by means of a time reference, which is received by the individual transmitter, for example, via GPS. Frequency synchronisation in the circuit arrangement according to DE 43 41 211 C1 finally takes place before modulation. A retrospective frequency displacement of the carrier frequency by subsequent functional units of the transmitter is therefore not excluded. All of these disadvantages can lead to an undesirable reception of different carrier frequencies of the individual transmitters in a receiver positioned anywhere within the transmission range of the single-frequency network.